Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• getMin() -- Retrieve the minimum element in the stack.

Have you met this question in a real interview?

思路1：用两个stack

class MinStack {

    Stack<Integer> minstack;
    Stack<Integer> stack;
    /** initialize your data structure here. */
    public MinStack() {
        this.minstack = new Stack<Integer>();
        this.stack = new Stack<Integer>();
    }
    
    public void push(int x) {
        stack.push(x);
        if(minstack.isEmpty() || minstack.peek() >= x) {
            minstack.push(x);
        }
    }
    
    public void pop() {
        int peekvalue = stack.pop();
        if(peekvalue == minstack.peek()) {
            minstack.pop();
        }
    }
    
    public int top() {
        return stack.peek();
    }
    
    public int getMin() {
        return minstack.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */

思路2：用linked list来模拟这个stack，node --> node，定义一个node class来每次维护这个min，这样也是const time。

class MinStack  {
    Node top = null;
 
    public void push(int x) {
         if(top==null){
             Node temp = new Node(x);
             temp.min = x;
             top = temp;
         }else{
             Node temp = new Node(x);
             temp.min = Math.min(top.min, x);
             temp.next = top;
             top = temp;
         }
    }

    public void pop() {
         top = top.next;
         return;
    }

    public int top() {
      return top == null ? 0: top.val;
    }

    public int getMin() {
      return top == null ? 0: top.min;
    }
    
    public class Node{
        int val ;
        Node next;
        int min;
        
        public Node(int val){
            this.val=val;
        }
    }
}